This invention relates to a non-contact vision based inspection system for flat specular parts such as ink jet printer nozzle plates.
Ink jet nozzle plate production includes the step of inspecting each nozzle for morphological defects such as nicks, cuts, or blockages. Defective nozzles result in compromised print quality. Once the nozzle plates are integrated into the print head assembly, print testing can be used to verify print head performance but inspection at this stage in the production process is often imprecise and requires, upon the detection of an improperly manufactured nozzle plate, the time consuming and expensive rework required to replace the defective nozzle plate.
Traditionally, the nozzle plates were not inspected directly and only tested through print testing after integration into print head assemblies. In other cases, a human operator would view the nozzle plate under a microscope and decide whether the nozzle plate passed or failed based on subjective, visual criteria. More recently, inspection by human observers has been augmented by the use of automated inspection methods. Still, these methods rely heavily on human observers for final go/no-go determination and there is an inflated false failure rate associated with these current automated inspection methods and systems. There is currently no high speed automated process which accurately inspects ink jet nozzle plates for morphological defects. One current inspection system includes the use of a 2-D CCD array camera as the primary imaging input device. Ink jet nozzle plates are small, generally long and skinny and have over a thousand very small, e.g. 301 xcexcm nozzle orifices and thus require high magnification for adequate assessment. Using a 2-D CCD array camera poses a variety of challenges. Since the captured image size is limited by the size of the array, the camera needs to be stepped across the part surface in order to make a complete assessment of the surface characteristics. Even using this system, challenges such as illumination, depth of focus, part orientation, and image capture speed have resulted in falsely inflated part rejection rates. The use of the 2-D CCD array camera based system involves a very time consuming inspection process riddled with potential sources for error from focusing, illumination, and placement. The 2-D CCD array camera must be focused at each step since the depth of focus is prohibitively small for high magnification, large field of view circumstances. The illumination angle is also critical since nozzle plate materials are most often highly specular and difficult to image. Also, heat is an issue which may cause damage to a nozzle plate due to mismatched coefficients of thermal expansion of materials used in manufacturing ink jet heads. Accordingly, illumination sources which are heat intensive can not be used. Also, illumination uniformity is a significant challenge for use with 2-D cameras as illumination levels often fall off towards the perimeter of the camera causing undesired perturbations in the resulting data.
Assuming that both illumination and focus are controlled, the composite image in the prior 2-D CCD camera based system is stitched together from the separate images collected at each focusing step and reconstructed in a single buffer in order to be analyzed. This process results in another potential error along the stitching boundaries where adjacent fields are abutted. Any variations in the placement of the part or the camera results in incorrect, partial, or repeated data along the boundary lines which makes measurements very difficult. In order to eliminate the stitching operations, each image may be inspected separately resulting in a severe increase in the inspection time and cost. The use of a 2-D CCD array camera is also quite wasteful in terms of processing time as only approximately 5% of the useable elements are actually used for the nozzle array inspection. The rest of the array elements image unimportant peripheral areas of the nozzle plate away from the nozzle orifices.
It is therefore an object of this invention to provide a non-contact vision based inspection system for flat specular parts.
It is a further object of this invention to provide such a system which more accurately inspects flat specular parts such as ink jet print head nozzle plates automatically and very quickly.
It is a further object of this invention to provide such a system which produces a single image of an entire ink jet print head nozzle plate thus eliminating the problems associated with stitching together individual images to form a composite image.
It is a further object of this invention to provide such a system which does not require refocusing of the imaging camera as it scans the part under inspection.
The invention results from the realization that the fine details (e.g. the 30 xcexcm nozzle orifices) of flat specular parts (e.g. nozzle plates) can be more accurately imaged and inspected not by taking successive images of each portion of the part, refocusing the imaging camera prior to capturing each image, and then stitching together the successive images to form a composite image of the whole part but instead by ensuring that before any image is taken the image plane defined by the imaging camera is rendered parallel to the object plane defined by the part and then using a linescan camera to scan the length of the part to capture a high quality single image of the whole part without the need to refocus the camera during the scan.
This invention features a non-contact vision based inspection system for flat specular parts, the system comprising an imaging camera defining an image plane and aimed at a part to be inspected for taking a single image of the part; a staging apparatus for moving the part relative to the camera until the single image of the part is obtained; and means for ensuring that the image plane remains parallel to the object plane defined by the part as the part moves relative to the imaging camera.
The imaging camera is preferably a linescan camera. The part is typically located on a translatable table and the camera is fixed in place with respect to the translation direction of the table.
The means for ensuring that the image plane and the object plane are parallel may include a goniometer between the part and the translation table for rotating the orientation of the part relative to the image plane. An optical source and a detector measure the distance between two points on the part and the image plane. The goniometer is located on the staging apparatus. A controller is programmed to be responsive to the optical source, the detector, and the goniometer for adjusting the orientation of the part until the object plane is parallel to the image plane.
Further included may be means for adjusting the distance between the image plane relative to the object plane for focusing the imaging camera. A magazine is typically disposed on the staging apparatus for holding a plurality of long narrow parts to be inspected in sequence. The system is particularly suited to the inspection of ink jet printer nozzle plates.
This invention also features a non-contact vision based inspection system for flat specular parts including ink jet nozzle plates. A linescan camera defines an imaging plane and is aimed at a part to be inspected for taking a single image of the part. A staging apparatus is provided for moving the part relative to the camera until the single image of the part is obtained. An optical source and a detector measure the distance between two points on the part and the image plane. A goniometer is disposed between the part and the staging apparatus. A controller is programmed to activate the goniometer in response to the optical source and the detector until the object plane defined by the part is parallel to the image plane.
A CCD array camera may be used and aimed at the part for locating a terminal end of the part. The controller is further programmed to activate the staging apparatus to move a terminal end of the part proximate the optical source, to activate the optical source and then again to activate the staging apparatus to move an opposite terminal end of the part proximate the optical source, and to again activate the optical source.
The method of inspecting flat specular parts in accordance with this invention includes placing a part in a spaced relationship with respect to an imaging camera, the part defining an object plane, the imaging camera defining an image plane; adjusting one of the object plane and the image plane until they are parallel to each other; and moving the imaging camera relative to the part until a single image of the part is recorded.
The method also typically comprises: locating an edge of the part; determining the location of the opposite edge of the part; calculating height difference between two edges of the part; adjusting the orientation of the part until the height difference between the two edges of the part is below a predetermined threshold; focusing a linescan camera at one edge of the part; scanning the part with the linescan camera until the full part is imaged with the single image; and using the image of the part as input to an inspection algorithm to inspect the part for defects.
Another feature of this invention is an imaging head for a non-contact vision based inspection system, the imaging head comprising an optical housing including a mount for a linescan camera and a 2D CCD array camera; a light source; means for directing light from the light source to a part to be inspected; means for directing light reflected from the part simultaneously to both the linescan camera and the 2D CCD camera; and a distance measuring device attached to the housing for measuring the orientation of image plane defined by the linescan camera relative to the object plane defined by the part. The means for directing light reflected from the part typically includes a beam splitter in the optical housing positioned to direct light to both the linescan CCD camera and 2D CCD array camera.